Recently, the technique of processing materials using laser beam energy has been developing gradually to cover the field of micro-processing.
In the processing technique using a mask pattern, the processable length has been extended to the range of nanometers that are further shorter than micrometers due to the progress of the patterning technique and the shortening of a laser wavelength.
On the other hand, direct processing using a laser also has been practiced in the range of micrometers in the case of processing metal and organic matters such as polyimide as a result of the progress in shortening of the wavelength and pulse width of laser beams.
Furthermore, processing such as perforating using a laser is shifting to ablation processing from thermal processing. Ablation is a phenomenon that the state of a material of an irradiation part shifts from melting to evaporation in a short period of time through irradiation of laser beams whose pulse width is very narrow. The level of thermal influence on the periphery of the beam irradiation part varies with an increase or a decrease in pulse width. In the case of processing using an ultrashort pulse laser that completes beam irradiation before thermal diffusion occurs, it is possible to make precise minute holes, with hardly any heat-affected layer being generated.
However, many of the lasers that are used for actual processing employ pulse widths of at least the nanosecond order, which makes it impossible to avoid the thermal influence. Hence, a photochemical reaction that is caused by ultraviolet rays is employed. Short-wavelength beams of a laser such as, for instance, an excimer laser have great energy per photon and therefore can cut a chemical bond that forms a molecular skeleton.
As described above, the selection of, for instance, the pulse width and wavelength of a laser beam to be used for irradiation conventionally has made it possible to carry out micro-processing. The study, however, has not progressed so much from the viewpoint of improvement of materials to be irradiated with a laser. Processing of glass that is a transparent material is important for optical application. In order to provide a glass suitable for laser processing, JP11(1999)-217237A proposes a technique for providing glass in which cracks tend not to occur by decreasing the laser processing threshold value through the introduction of silver into glass by ion exchange.
Glass containing a large amount of alkali metal allows silver ions to be introduced thereinto by a silver ion exchange. However, a phenomenon occurs in which silver ions are reduced in the vicinity of the glass surface and thereby are inhibited from differing into the glass. Hence, the effective laser processing region is limited to the vicinity of the glass surface and therefore it still is difficult to carry out processing of glass including processing of the inner part thereof, for instance, processing to make a through hole in a glass sheet. In addition, there is another problem that the ion exchange rate is low and it therefore is difficult to allow ions to reach the inner part of glass.
Moreover, glass for laser processing produced through the silver ion exchange contains a large amount of alkali metal or alkaline-earth metal and therefore has a high thermal expansion coefficient, which is a problem. Since heat is generated in a laser irradiation part during laser processing, the laser irradiation part and the vicinity thereof are deformed due to the stress caused by the difference in thermal expansion between them. When the thermal expansion coefficient of the glass is high, the size of a processing part measured during laser irradiation varies from that measured after laser irradiation. Hence, the dimensional accuracy of the processing part may deteriorate.
Furthermore, with respect to optical elements, it generally is desirable that the dimensional change that is caused by a temperature change be small. There also is a problem that such a dimensional change as described above may cause variations in the characteristics of optical elements.